The Theory of Natural Selection. 261 



external conditions of life) of almost every organic 

 type alters more or less from century to century — 

 whether from astronomical, geological, and geographi- 

 cal changes, or from the immigrations and emigrations 

 of other species living on contiguous areas, and so 

 on — it follows that the process of natural selection 

 need never reach a terminal phase. And forasmuch 

 as natural selection may thus continue, ad infi/iifiim, 

 slowly to alter a specific type in adaptation to a 

 gradually changing environment, if in any case the 

 alteration thus effected is sufficient in amount to lead 

 naturalists to name the result as a distinct species, 

 it follows that natural selection has transmuted one 

 specific type into another. Similarly, by a continuation 

 of the process, specific types would become transmuted 

 into generic, generic into family types, and so on. Thus 

 the process is supposed to go on throughout all the 

 countless forms of life continuously and simultaneously 

 — the world of organic types being thus regarded as 

 in a state of perpetual, though gradual, flux. 



Now, the first thing we have to notice about this 

 theory is, that in all its main elements it is merely 

 a statement of observable facts. It is an observable 

 fact that in all species of plants and animals a very 

 much larger number of individuals are born than can 

 possibly survive. Thus, for example, it has been 

 calculated that if the progeny of a single pair of 

 elephants — which are the slowest breeding of animals 

 — were all allowed to reach maturity and propagate, 

 in 750 years there would be living 19,000,000 de- 

 scendants. Again, in the case of vegetables, if a 

 species of annual plant produces only two seeds a 



